Because it is wrong to cause injury to another human being. That's a moral standard that most people would choose to subscribe to, regardless of religion or the lack thereof. In certain circumstances, it may be necessary to cause injury, if only to prevent greater harm.
Well, if you're an atheist, then people are nothing more than "animate material", aren't they? What's wrong with a strong ape getting rid of a weak ape, especially if it's going to be a burden? Who cares? It's just a blob of flesh.
Oh, and you say that "most" people subscribe to this belief. What about those who don't? Who are you to judge them? You're just an "ape" like they are.
What if I chose not to subscribe? By what standard can you or anyone force me to submit.
If morality is nothing more than a set of rules man has reasoned to and forced upon by others, it has no bind on anyone who chooses to ignore.
If however morality is a set of rules set by a moral (un-corruptible, in-fallible) authority. A moral authority gained through authorship. Then what?
You cannot have it both ways, either
killing is wrong because the author of life says it is
or
killing is wrong because man has reasoned it to be so.
If you subscribe to the later I'd say that is fairly communist of you, or at least fascist. A body of men ruling morality from intellectualism or power.
I am a Free man and no reasoning of another will constrain me. I will not be kept from killing simply because society says it is wrong. Society has no hold on me, it has no Moral Authority over me.
Are you a free man or are you told what is right and wrong by the ruling society of men.
Not if they're just material. Kicking a human being is equivalent to kicking the wall or a chair to the doctrinaire materialist; i.e. being human has no special meaning.